Comparing Methods for Pairing Electrodes Across Ears With Cochlear Implants

Abstract

Objectives: Currently, bilateral cochlear implants (CIs) are independently programmed in clinics using frequency allocations based on the relative location of a given electrode from the end of each electrode array. By pairing electrodes based on this method, bilateral CI recipients may have decreased sensitivity to interaural time differences (ITD) and/or interaural level differences (ILD), two cues critical for binaural tasks. There are multiple different binaural measures that can potentially be used to determine the optimal way to pair electrodes across the ears. Previous studies suggest that the optimal electrode pairing between the left and right ears may vary depending on the binaural task used. These studies, however, have only used one reference location or a single bilateral CI user. In both instances, it is difficult to determine if the results that were obtained reflect a measurement error or a systematic difference across binaural tasks. It is also difficult to determine from these studies if the differences between the three cues vary across electrode regions, which could result from differences in the availability of binaural cues across frequency regions. The purpose of this study was to determine if, after experience-dependent adaptation, there are systematic differences in the optimal pairing of electrodes at different points along the array for the optimal perception of ITD, ILD, and pitch. Design: Data from seven bilateral Nucleus users was collected and analyzed. Participants were tested with ITD, ILD, and pitch-matching tasks using five different reference electrodes in one ear, spaced across the array. Comparisons were conducted to determine if the optimal bilateral electrode pairs systematically differed in different regions depending on whether they were measured based on ITD sensitivity, ILD sensitivity, or pitch matching, and how those pairs differed from the pairing in the participants’ clinical programs. Results: Results indicate that there was a significant difference in the optimal pairing depending on the cue measured, but only at the basal end of the array. Conclusion: The results suggest that optimal electrode pairings differ depending on the cue measured to determine optimal pairing, at least for the basal end of the array. This also suggests that the improvements seen when using optimally paired electrodes may be tied to the particular percept being measured both to determine electrode pairing and to assess performance, at least for the basal end of the array.

The Relationship Between Interaural Insertion-Depth Differences, Scalar Location, and Interaural Time-Difference Processing in Adult Bilateral Cochlear-Implant Listeners

“…In one exception to the broad ITD tuning reported in the literature, Staisloff and Aronoff (2021) presented data interpreted as showing narrow tuning on the order of 1-2 electrodes for seven subjects using Cochlear-brand devices. These narrow tuning estimates might reflect a combination of two methodological factors.…”

Letter to the Editor: Discussion of Measurement and Analysis Techniques to Estimate Interaural Place-of-Stimulation Mismatch for Binaural Perception, Re: Staisloff and Aronoff (2021). Comparing Methods for Pairing Electrodes Across Ears With Cochlear Implants, Ear Hear, 42, 1218–1227

“…The measurements might have had too much variability to reliably observe the true underlying BI-CI ITD tuning. Staisloff and Aronoff (2021) used linear step sizes of 100 or 25 μs, which are relatively small compared to many of their reported ITD thresholds. As noted by Levitt (1971, p. 470), "difficulties occur with very large or very small step sizes.…”

Considerations for Fitting Cochlear Implants Bimodally and to the Single-Sided Deaf

“…This problem can be reduced by first matching the latencies (see section “latency mismatch compensation” and “latency fitting”) or by using non-singular ITD values, such as a large range of different, fixed ITDs ( Bernstein et al, 2018 ), or dynamically varying ITDs ( Dirks et al, 2020 ). The important positive aspect of ITD sensitivity testing is that it appears not to be affected by plasticity as pitch ( Bernstein et al, 2021 ; Hu & Dietz, 2015 ; Staisloff & Aronoff, 2021 ), because it arises from presumably hard-wired binaural interaction at the level of the brainstem. Overall, as a binaural task, this method appears to be the most direct towards the question how to measure a mismatch to improve binaural processing (in contrast to CT imaging being the most precise), but is time consuming and presumably challenging for many - and impossible for some - bimodal subjects.…”

“…Apart from these important multiple possible procedural biases, there is an additional critical limitation of pitch matching: The brain appears to adapt the pitch percept for each electrode to the respected programmed frequency bands already within the first months after the first fitting (Reiss et al, 2015). Comparing different measurement techniques to determine the interaural frequency mismatch, in experienced bilateral as well as SSD-CI patients, also hints at such plasticity effects in electric place pitch (Bernstein et al, 2021;Staisloff & Aronoff, 2021). In this case, the pitch matching might be an appropriate approach only for newly implanted CI patients, but potentially misleading for experienced users.…”

“…Measuring ITD sensitivity while varying the place of stimulation in one ear builds on the finding that a tonotopically matching stimulation results in maximum ITD sensitivity (Nuetzel & Hafter, 1976). In bilateral CI users, this technique has been shown to produce the expected results Poon et al, 2009;Staisloff & Aronoff, 2021). Together with ILD sensitivity, it is arguably the most direct measure of interaural frequency mismatch, as long as the goal is to maximize this binaural sensitivity.…”